Zoom in to 100% (which is what Moderators do when analyzing an image), and the flaws will be evident. Focus on the eye isn't sharp enough; stopping down the lens would have brought more of the frog into focus; there is significant noise in the dark areas; a tighter crop might be warranted; there are spots that appear to be sensor dust or lens dirt.
I didn't say that the grass should be in focus, and the fact that it isn't doesn't detract from the image. But not much of the frog is in focus. You may be able to sharpen the focus on his eye by using Smart Sharpen in Photoshop. I have applied that filter on a few formerly rejected images and gotten them accepted.
Do the reviewers not understand camouflage?
I have no idea if the moderators understand camouflage, but they have a sense for colour noise:
and they know about chromatic aberration:
Both are errors that can be worked on, especially when shooting RAW.
I don't retouch my photos at all.
There is probably no photographer who does not do some retouching work. It's not that we absolutely want to photoshop Trotsky out of the picture, but one or the other dust particle, denoising and sharpening, dodge and burn, and getting rid of chromatic aberration are quite noble editing tasks.
Noise unavoidable on an EOS 20D
Noise is everywhere, also on my 5DMark4, that's why you need to denoise. I also had a 20D and the noise reaction is good. I agree it is not as good as with my 5D, but it's workable.
The chromatic aberration is visible in the screenshot of mine. The biggest problem in your hardware is not the camera, but the lens.
A leaf is in the way across the frog's face that is out of focus. You should either have it in focus, or shoot at an angle to avoid it. Too much of the frog is out of focus. You should have tried to get the entire frog in focus, or zoom in on its face if that;s what you wanted, but all your subject must be in focus. An item cannot be subject and background at the same time. The other issues are covered.
Photographer and Nutrition Author
The majority of your image is out of focus. Blurry images won't sell.
See links below.
Better luck next time. 🙂
Adobe Stock is not a gallery. "Artistic effect" is desirable but only in otherwise perfect photos.